Multiple material systems for selective beam sintering

ABSTRACT

A method and apparatus for selectively sintering a layer of powder to produce a part comprising a plurality of sintered layers. The apparatus includes a computer controlling a laser to direct the laser energy onto the powder to produce a sintered mass. The computer either determines or is programmed with the boundaries of the desired cross-sectional regions of the part. For each cross-section, the aim of the laser beam is scanned over a layer of powder and the beam is switched on to sinter only the powder within the boundaries of the cross-section. Powder is applied and successive layers sintered until a completed part is formed. Preferably, the powder comprises a plurality of materials having different dissociation or bonding temperatures. The powder preferably comprises blended or coated materials.

BACKGROUND OF THE INVENTION

The present application is a continuation of copending application Ser.No. 07/951,349, filed Sep. 25, 1992, now U.S. Pat. No. 5,296,062, issuedMar. 22, 1994, which is a continuation of application Ser. No.07/814,715, filed Dec. 30, 1991, now abandoned, which is a continuationof copending application 07/559,338, filed Jul. 30, 1990, now U.S. Pat.No. 5,076,869, issued Dec. 31, 1991, which is a continuation ofapplication Ser. No. 07/402,694 filed Sep. 5, 1989, now U.S. Pat. No.4,944,817 issued Jul. 3, 1990, which is a continuation-in-part of Ser.No. 06/920,580, filed Oct. 17, 1986, now U.S. Pat. No. 4,863,538.

1. Field of the Invention

This invention relates to a method and apparatus which uses a directedenergy beam to selectively sinter a powder to produce a part. Inparticular, this invention relates to a computer aided laser apparatuswhich sequentially sinters a plurality of powder layers to build thedesired part in a layer-by-layer fashion. The present application isparticularly directed towards a powder comprising a plurality ofmaterials where the powder has more than one bonding or dissociationtemperature.

2. Description of the Relevant Art

The economies associated with conventional part production methods aregenerally related directly to the quantity of parts to be produced andthe desired material characteristics of the finished parts. For example,large scale manufacture casting and extrusion techniques are often costeffective, but these production methods are generally unacceptable forsmall quantities--i.e. replacement parts or prototype production. Manysuch conventional part production methods require expensive partspecific tooling. Even powder metallurgy requires a die for shaping thepowder, making powder metallurgy unattractive as a method for producinga small number of parts.

Where only a small number of parts are desired, conventional productionmethods involving a subtractive machining method are usually used toproduce the desired part. In such substractive methods, material is cutaway from a starting block of material to produce a more complex shape.Examples of substractive machine tool methods include: milling,drilling, grinding, lathe cutting, flame cutting, electric dischargemachine, etc. while such conventional machine tool substractive methodsare usually effective in producing the desired part, they are deficientin many respects.

First, such conventional machine tool substractive methods produce alarge amount of waste material for disposal. Further, such machine toolmethods usually involve a large initial expense for setting up theproper machining protocol and tools. As such, the set-up time is notonly expensive, but relies a great deal on human judgment and expertise.These problems are, of course, exacerbated when only a small number ofparts are to be produced.

Another difficulty associated with such conventional machiningtechniques involves tool wear--which not only involves the cost ofreplacement, but also reduces machining accuracy as the tool wears.Another limit on the accuracy and tolerance of any part produced byconventional machining techniques is the tolerance limits inherent inthe particular machine tool. For example, in a conventional millingmachine or lathe, the lead screws and ways are manufactured to a certaintolerance, which limits the tolerances obtainable in manufacturing apart on the machine tool. Of course, the tolerances attainable arereduced with age of the machine tool.

The final difficulty associated with such conventional machine toolsubtractive processes is the difficulty or impossibility of making manypart configurations. That is, conventional machining methods are usuallybest suited for producing symmetrical parts and parts where only theexterior part is machined. However, where a desired part is unusual inshape or has internal features, the machining becomes more difficult andquite often, the part must be divided into segments for production. Inmany cases, a particular part configuration is not possible because ofthe limitations imposed upon the tool placement on the part. Thus, thesize and configuration of the cutting tool do not permit access of thetool to produce the desired configuration.

There are other machining processes which are additive, for example,plating, cladding, and some welding processes are additive in thatmaterial is added to a starting substrate. In recent years, otheradditive-type machining methods have been developed which use a laserbeam to coat or deposit material on a starting article. Examples includeU.S. Pat. Nos. 4,117,302; 4,474,861; 4,300,474; and 4,323,756. Theserecent uses of lasers have been primarily limited to adding a coating toa previously machined article. Often such laser coating methods havebeen employed to achieve certain metallurgic properties obtainable onlyby such coating methods. Typically, in such laser coating methods thestarting article is rotated and the laser directed at a fixed locationwith the coating material sprayed onto the article so that the laserwill melt the coating onto the article.

SUMMARY OF THE INVENTION

The problems outlined above are in large major solved by the method andapparatus of the present invention. The present invention includes adirected energy beam--such as a laser--and is adaptable to producealmost any three dimensional part. The method of the present inventionis an additive process, with the powder being dispensed into a targetarea where the laser selectively sinters the powder to produce asintered layer. The invention is a layer-wise process in which thelayers are joined together until the completed part is formed. Themethod of the present invention is not limited to a particular type ofpowder, but rather is adaptable to plastic, metal, polymer, ceramic,wax, semiconductor or amorphous powders, or composite material powders.

Broadly speaking, the apparatus includes a laser or other directedenergy source which is selectable for emitting a beam in a target areawhere the part is produced. A powder dispenser system deposits powderinto the target area. A laser control mechanism operates to move the aimof the laser beam and modulates the laser to selectively sinter a layerof powder dispensed into the target area. The control mechanism operatesto selectively sinter only the powder disposed within defined boundariesto produce the desired layer of the part. The control mechanism operatesthe laser to selectively sinter sequential layers of powder, producing acompleted part comprising a plurality of layers sintered together. Thedefined boundaries of each layer correspond to respectivecross-sectional regions of the part. Preferably, the control mechanismincludes a computer--e.g. a CAD/CAM system--to determine the definedboundaries for each layer. That is, given the overall dimensions andconfiguration of the part, the computer determines the definedboundaries for each layer and operates the laser control mechanism inaccordance with the defined boundaries. Alternatively, the computer canbe initially programmed with the defined boundaries of each layer.

In a preferred form, the laser control mechanism includes a mechanismfor directing the laser beam in the target area and a mechanism formodulating the laser beam on and off to selectively sinter the powder inthe target area. In one embodiment, the directing mechanism operates tomove the aim of the laser beam in a continuous raster scan of targetarea. The modulating mechanism turns the laser beam on and off so thatthe powder is sintered only when the aim of the laser beam is within thedefined boundaries for the particular layer. Alternatively, thedirecting mechanism aims the laser beam only within the definedboundaries for the particular layer so that the laser beam can be lefton continuously to sinter the powder within the defined boundaries forthe particular layer.

In a preferred embodiment, the directing mechanism moves the laser beamin a repetitive raster scan of the target area using a pair of mirrorsdriven by galvonometers. The first mirror reflects the laser beam to thesecond mirror which reflects the beam into the target area. Shiftingmovement of the first mirror by its galvonometer shifts the laser beamgenerally in one direction in the target area. Similarly, shiftingmovement of the second mirror by its galvonometer shifts the laser beamin the target area in a second direction. Preferably, the mirrors areoriented relative to each other so that the first and second directionsare generally perpendicular to each other. Such an arrangement allowsfor many different types of scanning patterns of the laser beam in thetarget area, including the raster scan pattern of the preferredembodiment of the present invention.

The method of part production of the present invention includes thesteps of depositing a first portion of powder onto a target surface,scanning the aim of a directed energy beam (preferably a laser) over thetarget surface, and sintering a first layer of the first powder portionon the target surface. The first layer corresponds to a firstcross-sectional region of the part. The powder is sintered by operatingthe directed energy source when the aim of the beam is within theboundaries defining the first layers. A second portion of powder isdeposited onto the first sintered layer and the aim of the laser beamscanned over the first sintered layer. A second layer of the secondpowdered portion is sintered by operating the directed energy sourcewhen the aim of the beam is within the boundaries defining the secondlayer. Sintering of the second layer also joins the first and secondlayers into a cohesive mass. Successive portions of powder are depositedonto the previously sintered layers, each layer being sintered in turn.In one embodiment, the powder is deposited continuously into the target.

In a preferred embodiment, the laser beam is modulated on and off duringthe raster scan so that the powder is sintered when the aim of the beamis directed within the boundaries of the particular layer. Preferably,the laser is controlled by a computer; the computer may include aCAD/CAM system, where the computer is-given the overall dimensions andconfiguration of the part to be made and the computer determines theboundaries of each cross-sectional region of the part. Using thedetermined boundaries, the computer controls the sintering of each layercorresponding to the cross-sectional regions of the part. In analternative embodiment, the computer is simply programmed with theboundaries of each cross-sectional region of the part.

Additionally, another embodiment of the present invention includes apowder comprising a plurality of materials where the plurality ofmaterials have more than one dissociation temperature. In still anotherembodiment of the present invention, the powder comprises a plurality ofmaterials where the plurality of materials have more than one bondingtemperature.

As used throughout this document, bonding temperature includes but isnot limited to, melting temperature, softening temperature and bondingtemperature.

In all preferred embodiments of the present invention, the plurality ofmaterials comprise at least one first material blended with at least onesecond material or at least one first material coated with at least onesecond material.

As can be appreciated from the above general description, the method andapparatus of the present invention solves many of the problemsassociated with known part production methods. First, the presentinvention is well suited for prototype part production or replacementpart production of limited quantities.. Further, the method andapparatus hereof are capable of making parts of complex configurationsunobtainable by conventional production methods. Further, the presentinvention eliminates tool wear and machine design as limiting factors onthe tolerances obtainable in producing the part. Finally, with theapparatus of the present invention incorporated into a CAD/CAMenvironment, a large number of replacement parts can be programmed intothe computer and can be easily produced with little set-up or humanintervention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the apparatus of the presentinvention;

FIG. 2 is a schematic showing a portion of the layered build up of apart produced in accordance with the present invention, and illustratingthe raster scan pattern of the laser beam in the target area;

FIG. 3 is a block diagram depicting the interface hardware between thecomputer, laser and galvonometers-of the present invention;

FIG. 4 is a perspective view of an example part produced in accordancewith the present invention;

FIG. 5 is a sectional view with parts broken away and in phantom, of thepart illustrated in FIG. 4;

FIG. 6 is a flow chart of the data metering program in accordance withthe present invention;

FIG. 7 is a sectional view taken along line 7--7 of FIG. 4;

FIG. 8 illustrates in diagram form the correlation between a singlesweep of the laser across the layer of FIG. 7 and the control signals ofthe present invention;

FIG. 9 illustrates a blend of materials in a powder;

FIG. 10 illustrates coated materials in a powder;

FIG. 11a-11d illustrates a portion of a sintering cycle on a blend ofmaterials as presently understood.

FIG. 12 illustrates two materials deposited prior to sintering.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 broadly illustrates the apparatus 10in accordance with the present invention. Broadly speaking, theapparatus 10 includes a laser 12, powder dispenser 14, and laser controlmeans 16. In more detail, the powder dispenser 14 includes a hopper 20for receiving the powder 22 and having an outlet 24. The outlet 24 isoriented for dispensing the powder to a target area 26, which in FIG. 1is generally defined by the confinement structure 28. Of course, manyalternatives exist for dispensing the powder 22.

The components of the laser 12 are shown somewhat schematically in FIG.1 and include a laser head 30, a safety shutter 32, and a front mirrorassembly 34. The type of laser used is dependent upon many factors, andin particular upon the type of powder 22 that is to be sintered. In theembodiment of FIG. 1, a Nd:YAG laser (Lasermetrics 9500Q) was used whichcan operate in a continuous or pulsed mode with a hundred-watt maximumoutlet power in the continuous mode. The laser beam output of the laser12 has a wavelength of approximately 1060 run, which is near infrared.The laser 12 illustrated in FIG. 1 includes an internal pulse rategenerator with a selectable range of about one kilohertz to fortykilohertz, and an approximately six nanosecond pulse duration. In eitherthe pulsed or continuous mode, the laser 12 can be modulated on or offto selectively produce a laser beam which travels generally along thepath shown by the arrows in FIG. 1.

To focus the laser beam, a diverging lens 36 and converging lens 38 aredisposed along the path of travel of the laser beam as shown in FIG. 1.Using Just the converging lens 38, the location of the true focal pointis not easily controlled by varying the distance between the converginglens 38 and the laser 12. The diverging lens 36 placed between the laser12 and converging lens 38 creates a virtual focal point between thediverging lens 36 and the laser 12. Varying the distance between theconverging lens 38 and the virtual vocal point, allows control of thetrue focal point along the laser beam path of travel on the side of theconverging lens 38 remote from the laser 12. In recent years there havebeen many advances in the field of optics, and it is recognized thatmany alternatives are available to efficiently focus the laser beam at aknown location.

In more detail, the laser control means 16 includes computer 40 andscanning system. 42. In a preferred embodiment, the computer 40 includesa microprocessor for controlling the laser 12 and a CAD/CAM system forgenerating the data. In the embodiment illustrated in FIG. 1, a personalcomputer is used (Commodore 64) whose primary attributes include anaccessible interface port and a flag line which generates a nonmaskableinterrupt.

As shown in FIG. 1, the scanning system 42 includes a prism 44 forredirecting the path of travel of the laser beam. Of course, physicallayout of the apparatus 10 is the primary consideration in determiningwhether a prism 44, or a plurality of prisms 44, are needed tomanipulate the path of travel of the laser beam. The scanning system 42also includes a pair of mirrors 46, 47 driven by respectivegalvonometers 48, 49. The galvonometers 48, 49 coupled to theirrespective mirrors 46, 47 to selectively orientate the mirrors 46, 47.The galvonometers 46, 47 are mounted perpendicular to each other suchthat the mirrors 46, 47 are mounted nominally at a right angle to eachother. A function generator driver 50 controls the movement of thegalvonometer 48 (galvonometer 49 is slaved to the movement ofgalvonometer 48) so that the aim of the laser beam (represented by thearrows in FIG. 1) can be controlled in the target area 26. The driver 50is operatively coupled to the computer 40 as shown in FIG. 1. It will beappreciated that alternative scanning methods are available for use asthe scanning system 42, including acusto-optic scanners, rotatingpolygon mirrors, and resonant mirror scanners.

Turning to FIG. 2 of the drawing, a portion of a part 52 isschematically illustrated and shows four layers 54-57. The aim of thelaser beam; labeled 64 in FIG. 2, is directed in a raster scan patternas at 66. As used herein, "aim" is used as a neutral term indicatingdirection, but does not imply the modulation state of the laser 12. Forconvenience, the axis 68 is considered the fast scan axis, while theaxis 70 is referred to as the slow scan axis. Axis 72 is the directionof part build-up.

Turning to FIGS. 9 and 10, powders, comprising a plurality of materialsby which parts may be made using the present invention, are illustrated.For simplicity, only two materials are shown in the illustrations.However, as will be apparent to one skilled in the art, a plurality ofmaterials may comprise the powder of the present invention.

FIG. 9 illustrates a blend of first material 901 and second material902. The materials are combined in a blend through conventional blendingprocesses. FIG. 10 illustrates material 1002 coated in material 1001.Material 1002 is coated using conventional coating processes.

As will be further apparent to one skilled in the art, coated materials,as shown in FIG. 10, may be blended to produce a desired mix ofmaterials.

Turning to FIG. 11, a portion of a sintering cycle, as presentlyunderstood, is illustrated. FIG. 11a illustrates a blend of materialsprior to the application of energy able to produce sintering.Preferably, the materials comprising powder mass 1100 have more than onebonding or dissociation temperature. FIG. 11b illustrates powder 1100during application of energy sufficient to promote sintering. FIG. 11billustrates material 1101 having a lower bonding or dissociationtemperature than material 1102. In a preferred embodiment, the lowtemperature phase material 1101 melts and infiltrates powder mass 1100in the area surrounding each particle of material 1101. Additionalpowder components could also be added to the blend to promoteinfiltration. Similarly a gas phase can be used to promote infiltrationand the sintering process. The gas phase may be either inert or active,preferably to either displace an undesired gas or introduce a desiredgas. FIG. 11c illustrates a potential mechanism through which effects,including but not limited to, capillarity effects, allow material 1101to infiltrate the powder mass 1100. FIG. 11d shows the part followingsintering in the present invention.

Because a material having a bonding or dissociation temperature higherthan the temperature to be obtained during the sintering process may beselected, the higher bonding or dissociation temperature material neednot sinter but may retain its original structure. In particular, forcrystalline material this enables control of epitaxial growth in theselective beam sintering process of the present invention. For example,if the higher bonding or dissociation temperature material is positionedin a particular structure that may, preferably, result in epitaxialgrowth from the preceding layer, only bonding or dissociating the lowerbonding or dissociation temperature material enables the highertemperature material to retain its structure.

The choice of materials selected for the powder allows for a broad rangeof resulting sintered material. For example, a conducting material ispreferably coated with an insulating polymer material to produce apowder. The powder is then distributed in the target area. The materialis preferably sintered and the insulator may be removed later through aconventional process, including but not limited to a chemical method,resulting in a conductive, sintered product.

By way of further illustration, extremely hard materials may be producedusing the present invention. For example, tungsten carbide/cobalt toolswhich, because of their extreme hardness are difficult to form orsharpen may be produced by coating tungsten carbide material with cobaltto produce a powder or by blending tungsten carbide and cobalt toproduce a powder. During sintering, the cobalt preferably melts underthe applied energy beam causing local infiltration of the tungstencarbide. The part that is manufactured is ready to be put into servicepreferably after a secondary process including, but not limited to,annealing.

By way of further illustration, copper and tin may be combined in apowder. Tin, having a lower melting temperature than copper, will meltand infiltrate the copper during sintering.

Secondary processing may also be applied to parts produced using thepresent invention. For example, where tin is allowed to melt andinfiltrate copper during sintering, post process annealing will dissolvethe tin into the copper in the solid state creating bronze with minimalvolume change or distortion.

By way of further illustration, metal, including but not limited to,iron or steel, may be coated with poly(methyl methacrylate) (PMMA)polymer to form a powder. Sintering enables the PMMA to flow and bindthe metal. Post process annealing will dissociate the PMMA and sinterthe metal thus producing a final part.

Ceramic materials may be processed in this fashion as well. For example,a mixture of fluorophosphate glass powders with alumina powders willresult in the glass softening and infiltrating the alumina during thesintering process. In another example, aluminum silicate, silica, orother ceramic powder can be coated with a polymer by a variety ofmethods, including spray drying and solvent coating. A surface activeagent may be used to pretreat the ceramic powder prior to coating. Thisagent may be based on organosilane chemistry or other chemistries knownto promote the wetability of the ceramic by the polymer and the adhesionof ceramic to polymer. Any polymer, either thermoplastic or thermoset,which can be coated on the ceramic, can be used as a binder. Typicalmaterials include PMMA, polystyrene, various epoxy formulations, andphenolics.

Any combination of materials, including but not limited to, metals,ceramics and polymers enables production of parts in accordance with thepresent invention wherein at least one material in the powder has a lowbonding or dissociation temperature relative to the other materials inthe powder.

In another preferred embodiment of the present invention, thetemperature of the powder mass may be increased using conventionalheating means allowing the energy beam to merely supply a small increaseof energy to produce bonding or dissociation of one of the elementalmaterials of the powder.

Materials comprising the powder may be chosen for each material'sselective absorption of energy from a laser beam (represented by thearrows in FIGS. 11a and 11b). In the preferred embodiment shown in FIG.11, material 1101 may be chosen to absorb the wavelength of the appliedbeam energy represented by the arrows while elemental material 1102absorbs less energy thereby enabling elemental material 1101 to bond ordissociate prior to the bonding or dissociation of elemental material1102. This absorption of energy can be achieved by either material orlaser beam wavelength selection, or both, in a plurality ofcombinations.

Turning to FIG. 12, in yet another preferred embodiment a material 1201is preferably deposited on surface 1200 and second material 1203 is thendeposited on material 1201 prior to sintering. Materials 1201 and 1203preferably have different bonding or dissociation temperatures.

Operation

A fundamental concept of the present invention is the build up of a partin a layer-by-layer manner. That is, a part is considered a plurality ofdiscrete cross-sectional regions which cumulatively comprise thethree-dimensional configuration of the part. Each discretecross-sectional region has defined two-dimensional boundaries--ofcourse, each region may have unique boundaries.

In the method, a first portion of powder 22 is deposited in the targetarea 26 and selectively sintered by the laser beam 64 to produce a firstsintered layer 54 (FIG. 2). The first sintered layer 54 corresponds to afirst cross-sectional region of the desired part. The laser beamselectively sinters only the deposited powder 22 within the confines ofthe defined boundaries.

There are, of course, alternative methods of selectively sintering thepowder 22. One method is for the aim of the beam to be directed in a"vector" fashion--that is, the beam would actually trace the outline andinterior of each cross-sectional region of the desired part.Alternatively, the aim of the beam 64 is scanned in a repetitive patternand the laser 12 modulated. In FIG. 2, a raster scan pattern 66 is usedand is advantageous over the vector mode primarily in its simplicity ofimplementation. Another possibility is to combine the vector and rasterscan methods so that the desired boundaries of the layer are traced in avector mode and the interior irradiated in a raster scan mode. Thereare, of course, trade-offs associated with the method chosen. Forexample, the raster mode has a disadvantage when compared to the vectormode in that arcs and lines which are not parallel to the axes 68, 70 ofthe raster pattern 66 of the laser beam 64 are only approximated. Thus,in some cases resolution of the part can be degraded when produced inthe raster pattern mode. However, the raster mode is advantageous overthe vector mode in the simplicity of implementation.

Turning to FIG. 1, the aim of the laser beam 64 is scanned in the targetarea 26 in a continuous raster pattern. Broadly speaking, the driver 50controls galvonometers 48, 49 to made the raster pattern 66 (see FIG.2). Shifting movement of the mirror 46 controls movement of the aim ofthe laser beam 64 in the fast scan axis 68 (FIG. 2), while movement ofthe mirror 47 controls movement of the aim of the laser beam 64 in theslow scan access 70.

The present position of the aim of the beam 64 is fed back through thedriver 50 to the computer 40 (see FIG. 3). As described below, in moredetail, the computer 40 possesses information relating to the desiredcross-sectional region of the part then being produced. That is, aportion of loose powder 22 is dispensed into the target area 26 and theaim of the laser beam 64 moved in its continuous raster pattern. Thecomputer 40 modulates the laser 12 to selectively produce a laser beamat desired intervals in the raster pattern 66. In this fashion, thedirected beam of the laser 12 selectively sinters the powder 22 in thetarget area 26 to produce the desired sintered layer with the definedboundaries of the desired cross-sectional region. This process isrepeated layer-by-layer with the individual layers sintered together toproduce a cohesive part--e.g. part 52 of FIG. 2.

In operation, the wavelength of laser 12 may be varied to produce higherabsorptivity of energy by selected materials in the powder relative toother materials in powder 22. In operation, blended, coated or othercombinations of powders are preferably selected to produce sinteredproduct with characteristics including but not limited to, closedimensional tolerances, structural integrity and required mechanicalbehavior.

Interface and Software

The interface hardware operatively interconnects the computer 40 withthe laser 12 and galvonometers 47, 48. The output port of the computer40 (see FIGS. 1 and 3) is directly connected to the laser 12 toselectively modulate the laser 12. When operated in the pulsed mode, thelaser 12 is easily controlled by digital inputs to the pulsed gate inputof the laser. Galvonometer 48 is driven by the function generator driver50 to drive the beam in the fast scan axis 68 independent of any controlsignals from the computer 40. However, a position feedback signal fromthe galvonometer 48 is fed to a voltage comparator 74 as shown in FIG.3. The other input to the comparator is connected to thedigital-to-analog convertor 76 which is indicative of the leastsignificant six bits (bits 0-5) of the user port of the computer 40. Asshown in FIG. 3, the output of the voltage comparator 74 is connected tothe flag line on the user port of the computer 40. When the voltagecomparator determines that the feedback signal from the galvonometer 48crosses the signal from the digital-to-analog convertor 76, the flagline goes low causing a nonmaskable interrupt. As discussed below, thenonmaskable interrupt causes the next byte of data to put out on theuser port of a computer 40.

Finally, as shown in FIG. 3, the galvonometer 49 driving the aim of thelaser beam 64 in the slow scan axis 70, is controlled by a seconddigital to analog convertor 78. The digital-to-analog convertor 78 isdriven by a counter 79 which increments with each sweep of the aim ofthe beam 64 in the fast scan axis 68. The eight byte counter is designedto overflow after 256 scans in the fast scan axis 68 to start a newcycle or raster scan pattern 66.

Preferably, the control information (i.e. defined boundaries of thecross-sectional regions) data for each raster pattern 66 would bedetermined by a CAD system given the overall dimensions andconfiguration of the part to be produced. Whether programmed or derived,the control information data for each raster pattern 66 is stored in thecomputer memory as a series of eight bit words. The data formatrepresents a pattern of "on" and "off" regions of the laser 12, versusdistance along the raster pattern 66 traveled by the aim of the beam 64.The data is stored in a "toggle-point" format where the data representsthe distance along each raster scan pattern 66 where the laser ismodulated (i.e. turned from on to off or from off to on). Although a"bit map" format might be used, the toggle point format has been foundmore efficient for the production of high resolution parts.

For each eight bit word, the least significant six bits (bits 0-5)represent the location of the next toggle point--i.e. The next locationfor modulation of the laser 12. The next bit (bit 6) represents whetherthe laser is on or off immediately before the toggle point identified inthe least significant six bits. The most significant bit (MSB or bit 7)is used for looping and for controlling the slow scan axis 70 of the aimof the beam 64. Because the Commodore 64 had limited memory, looping wasrequired --it being understood that a computer 40 with more memory wouldnot require looping.

FIG. 6 represents the flow chart for the data metering program. The datametering program is run whenever the flagline goes low causing anon-maskable interrupt (see FIG. 3). The interrupt causes themicroprocessor of the computer 40 to retrieve a two byte interruptvector which points to the location in memory where program control istransferred at interrupt. As shown in FIG. 6, the data metering programfirst pushes the registers onto the stack and then loads the next byteof data into the accumulator. The data word is also output to the userport with the sixth bit used to modulate the laser 12 (FIG. 3).

As shown in FIG. 6, the most significant bit (MSB or bit 7) of the dataword in the accumulator is examined. If the value of the mostsignificant bit is one, that means the end of the loop has not beenreached; therefore the data pointer is incremented, registers arerestored from the stack, and the data metering program is exited,returning control to the microprocessor at the location of interrupt. Ifthe most significant bit in the accumulator is zero, the data word isthe last word in the loop. If the data word is the last word in theloop, the next bit in memory is a loop counter and the following twobytes are a vector pointing to the top of the loop. As can be seen fromFIG. 6, if the most significant bit equals zero (end of the loop) theloop counter (next bit) is decremented and analyzed if the loop counteris still greater than zero, the data pointer assumes the value from thenext two memory bytes after the loop counter, registers are pulled fromthe stack and program control returns to the location of interrupt. Onthe other hand, if loop counter is zero, the data pointer is incrementedby three and the loop counter is reset to ten before exiting theprogram. It can be appreciated that the need for such looping isabsolved if the memory size of the computer 40 is adequate.

EXAMPLE

In FIGS. 4 and 5, an example part 52 is illustrated. As can be seen fromthe drawing, the example part 52 assumes an unusual shape in that it isnot symmetrical and would be difficult to fabricate using conventionalmachining methods. For reference purposes, the part 52 includes an outerbase structure 80 having an interior cavity 82 and a pillar 84 disposedwithin the cavity 82 (see FIG. 4). FIG. 5 shows the part 52 within theconfinement structure 28 defining the target area 26 illustrated inFIG. 1. As shown in FIG. 5, some of the powder 22 is loose, while theremainder of the powder is selectively sintered to comprise thestructure of the part 52. FIG. 5 is shown in vertical section with partsbroken away and outlined in phantom to show the sintered cohesiveportions of the part 52.

FIG. 7 shows a horizontal cross-sectional region, taken along line 7--7of FIG. 4. FIG. 7 represents a discrete layer 86 associated with thecross-sectional region of the part being produced. As such, the sinteredlayer 86 of FIG. 7 is a product of a single raster pattern 66 asillustrated in FIG. 2.

For reference purposes, a sweep line through the sintered layer 86 hasbeen labeled "L." FIG. 8 illustrates the software and hardware interfaceoperation during the sweep L. The top graph shows the position offeedback signal from the fast axis galvo 48 and the output signal of thefirst digital to analog convertor 76 (compare FIG. 3). The voltagecomparator 74 generates an output signal to the flag line of thecomputer 40 every time the feedback signal and first D/A output signalcross.

In the top graph of FIG. 8, these points are labeled T to representtoggle points. As can be seen from the bottom graph of FIG. 8, the flagline generates a nonmaskable interrupt corresponding to each togglepoint T. The sixth bit of each data word is analyzed and the currentstate of the laser 12 will reflect the value. The penultimate graph ofFIG. 8 shows the laser modulation signal for the sweep line L of FIG. 7.The second graph of FIG. 8 shows that a high-going edge in the mostsignificant bit will be encountered at the end of each sweep of the aimof the laser beam 64 in the fast scan axis 68. As shown in FIGS. 3 and6, the counter 79 increments on a high going edge, and outputs a signalto the second digital-analog convertor 78 to drive the slow axisgalvonometer 49.

As can be seen from the example illustrated in the drawing, parts ofcomplex shape can be produced with relative ease. Those skilled in theart will appreciate that the part 52 illustrated in FIG. 4 would bedifficult to produce using conventional machining methods. Inparticular, machine tool access would make the fabrication of cavity 82and pillar 84 difficult, if not impossible, to produce if the part 52were of a relatively small size.

In addition to avoiding the access problem, it will be appreciated thatthe production accuracy is not dependent upon machine tool wear and theaccuracy of mechanical components found in conventional machine tools.That is, the accuracy and tolerances of the parts produced by the methodand apparatus of the present invention are primarily a function of thequality of the electronics, the optics, and the implementing software.Of course, heat transfer and material considerations do affect thetolerances obtainable.

Those skilled in the art will appreciate that conventional machiningtechniques require considerable human intervention and Judgment. Forexample, a conventional machining process, such as milling, wouldrequire creativity to make such decisions as tool selection, partsegmenting, sequence of cuts, etc. Such decisions would even be moreimportant when producing a control tape for a tape control millingmachine. On the other hand, the apparatus of the present invention onlyrequires the data relating to each cross-sectional region of the partbeing produced. While such data can be simply programmed into thecomputer 40, preferably, the computer 40 includes a CAD/CAM system. Thatis, the CAD/CAM portion of the computer 40 is given the overalldimensions and configurations of the desired part to be produced and thecomputer 40 determines the boundaries for each discrete cross-sectionalregion of the part. Thus, a vast inventory of part information can bestored and fed to the computer 40 on a selectable basis. The apparatus10 produces a selected part without set-up time, part specific tooling,or human intervention. Even the complex and expensive dies associatedwith powder metallargy and conventional casting techniques are avoided.

While large quantity production runs and certain part materialcharacteristics might be most advantageously made using conventionalfabrication techniques, the method and apparatus 10 of the presentinvention is useful in many contexts. In particular, prototype modelsand casting patterns are easily and inexpensively produced. For example,casting patterns are easily made for use in sand casting, lost waxcasting, or other forming techniques. Further, where desired quantitiesare very small, such as with obsolete replacement parts, production ofsuch replacement parts using the apparatus 10 of the present inventionhas many advantages. Finally, the use of the apparatus 10 may be usefulwhere size of production facilities is a major constraint, such ason-ship or in outerspace.

Further modification and alternative embodiments of the apparatus ofthis invention will be apparent to those skilled in the art in view ofthis description. Accordingly this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the manner of carrying out the invention. It is to be understoodthat the forms of the invention herein shown and described are to betaken as the presently preferred embodiments. Various changes may bemade in the shape, size and arrangement of parts. For example, elementsor materials may be substituted for those illustrated and describedherein, parts may be reversed, and certain features of the invention maybe utilized independently of the use of other features, all as would beapparent to one skilled in the art after having the benefit of thisdescription of the invention.

We claim:
 1. A method of producing a part, comprising the stepsof:depositing a layer of a powder onto a target surface, said powdercomprising particles of a first material coated with a second material,said second material having a lower bonding temperature than said firstmaterial; heating the deposited layer of powder to a temperature that isnear but below the bonding temperature of the second material;irradiating a selected portion of said powder corresponding to across-section of the part with a directed energy beam, so that thesecond material in said selected portion bonds particles of said firstmaterial; repeating said depositing, heating and irradiating steps for aplurality of layers so that bonded portions of adjacent layers bond toone another to form a mass; removing unbonded portions of the powder toyield the mass;
 2. The method of claim 1, further comprising:heating themass after said removing step.
 3. The method of claim 2, wherein saidsecond material is driven off in said heating step;and wherein saidheating step fuses particles of said first material to one another. 4.The method of claim 2, wherein said first and second materials interactin said heating step.
 5. The method of claim 1, wherein said secondmaterial comprises a polymer of the PMMA type.
 6. The method of claim 1,wherein the thermal conductivity of said first material is substantiallygreater than that of said second material.
 7. The method of claim 1,wherein said first and second materials have different absorptivitycharacteristics;and wherein the wavelength of the directed energy beamis selected so as to be more highly absorbed by said second materialthan by said first material.
 8. The method of claim 1, furthercomprising:during said irradiating step, exposing said powder to a gasphase to promote infiltration of the second material within particles ofthe first material;
 9. The method of claim 8, wherein said gas phase isinert.
 10. The method of claim 8, wherein said gas phase is active. 11.The method of claim 1, wherein said second material is a metal.
 12. Themethod of claim 11, wherein said first material is copper and saidsecond material is tin.
 13. The method of claim 11, wherein said firstmaterial is tungsten carbide and said second material is cobalt.